Patent application title: ULTRASOUND DIAGNOSTIC APPARATUS

Abstract:

A transmission wave corresponding to an FM continuous wave having been
subjected to an FM modulation processing is transmitted from a
transmitting transducer 10. A pre-amplifier 16 generates a reception RF
signal and outputs the reception RF signal to a receiving mixer 30. The
receiving mixer 30 applies orthogonal detection to the reception RF
signal to generate a complex signal. A reference signal supplied to each
mixer in the receiving mixer 30 is generated based on an FM continuous
wave output from an FM modulator 20. The FM continuous wave output from
the FM modulator 20 is delayed by a delay circuit 25, and one signal is
directly supplied to a mixer 32 whereas the other signal is supplied to a
mixer 34 via a π/2 shift circuit 26. The delay circuit 25 applies a
delay processing in accordance with a depth of a target position within a
living organism to the FM continuous wave. As a result, Doppler
information from the target position can be selectively extracted by a
Doppler information analyzing section 44.

Claims:

1. An ultrasound diagnostic apparatus comprising:a transmission signal
processing section which outputs a modulated transmission signal that is
a continuous wave having a frequency varied periodically;a
transmission/reception section which transmits ultrasound to a living
organism based on the modulated transmission signal and receives a
reflection wave from the living organism, thereby obtaining a reception
signal;a reception signal processing section which applies a demodulation
processing to the reception signal by using a reference signal having a
waveform that is substantially the same as a waveform of the modulated
transmission signal to obtain a demodulated signal; anda Doppler
information extraction section which extracts Doppler information from
the demodulated signal,whereina delay processing in accordance with a
depth of a target position within the living organism is performed to
adjust a delay relationship between the reference signal and the
reception signal and the demodulation processing is performed, thereby
selectively extracting the Doppler information from the target position.

2. The ultrasound diagnostic apparatus according to claim 1, whereinat
least one of the modulated transmission signal output from the
transmission signal processing section and the reference signal to be
used in the reception signal processing section is delayed to enhance a
correlation between the reception signal from the target position and the
reference signal, thereby selectively extracting the Doppler information
from the target position.

3. The ultrasound diagnostic apparatus according to claim 2, whereinthe at
least one of the modulated transmission signal and the reference signal
is delayed such that a phase of the reception signal from the target
position and a phase of the reference signal are identical.

4. The ultrasound diagnostic apparatus according to claim 3, whereinthe
modulated transmission signal or the reference signal is delayed by a
delay amount in accordance with a depth of the target position such that
a phase of the reception signal from the target position and a phase of
the reference signal are identical.

5. The ultrasound diagnostic apparatus according to claim 4, whereinthe
Doppler information is extracted from the target position while varying
the delay amount to thereby shift the target position along a depth
direction.

6. The ultrasound diagnostic apparatus according to claim 5, whereinthe
Doppler information is extracted from a plurality of positions through a
subject section along the depth direction, by periodically varying the
delay amount to thereby periodically shift the target position within the
subject section along the depth direction.

7. The ultrasound diagnostic apparatus according to claim 6, whereina
velocity distribution of a fluid within the subject section is formed
based on the Doppler information extracted from the plurality of
positions in the subject section.

8. The ultrasound diagnostic apparatus according to claim 7, whereina
display image which includes an axis corresponding to a depth within the
living organism, an axis corresponding to time, and an axis corresponding
to a velocity of the fluid and which shows a state of a variation in the
velocity distribution with time is formed.

9. The ultrasound diagnostic apparatus according to claim 1, whereinthe
Doppler information extraction section extracts, as the Doppler
information, a Doppler signal component corresponding to a direct current
signal component which is contained in the demodulated signal.

11. The ultrasound diagnostic apparatus according to claim 10, whereinthe
modulated transmission signal which is generated by the transmission
signal processing section is delayed by a delay amount in accordance with
a depth of the target position, to form the reference signal.

12. The ultrasound diagnostic apparatus according to claim 10, whereina
modulation signal to be used for the frequency modulation processing is
delayed by a delay amount in accordance with the depth of the target
position to obtain a delayed modulation signal, and the carrier wave
signal is subjected to the frequency modulation processing by using the
delayed modulation signal.

13. The ultrasound diagnostic apparatus according to claim 10, whereina
modulation index of the frequency modulation processing performed by the
transmission signal processing section is adjusted to set a position
resolution.

14. The ultrasound diagnostic apparatus according to claim 13, whereinthe
modulation index of the frequency modulation processing performed by the
transmission signal processing section is a ratio of a maximum frequency
deviation and a modulation frequency and is set to a value which is 1 or
greater.

15. The ultrasound diagnostic apparatus according to claim 13, whereinthe
modulation index of the frequency modulation processing performed by the
transmission signal processing section is a ratio of a maximum frequency
deviation and a modulation frequency and is set to a value which is 30 or
greater.

17. The ultrasound diagnostic apparatus according to claim 1, whereinthe
transmission signal processing section generates the modulated
transmission signal based on data of a continuous wave having a frequency
varied periodically.

Description:

BACKGROUND

[0001]1. Technical Field

[0002]The present invention relates to an ultrasound diagnostic apparatus,
and more particularly to an ultrasound diagnostic apparatus in which a
modulated continuous wave is utilized.

[0003]2. Related Art

[0004]Continuous wave Doppler is a known ultrasound diagnostic apparatus
technology in which a continuous wave is employed. In continuous wave
Doppler technology, a transmission wave which is formed as a sinusoidal
wave of several MHz is continuously radiated into a living organism and a
reflection wave from within the living organism is then continuously
received. The reflection wave includes Doppler shift information
generated by a moving element (e.g. blood flow) within the living
organism. Accordingly, by extracting the Doppler shift information and
applying frequency analysis to the Doppler shift information, a Doppler
waveform which reflects information of velocity of the moving element,
for example, can be formed.

[0005]Continuous wave Doppler technology in which a continuous wave is
utilized is generally superior to Pulse Doppler, in which a pulse wave is
utilized, for rapid acquisition of velocity measurements. Under such
circumstances, the inventors of the present application have conducted
research concerning continuous wave Doppler technology. In one of their
achievements, the present inventors proposed the technology concerning
Frequency Modulated Continuous wave Doppler (FMCW Doppler) disclosed in
JP 2005-253949 A.

[0006]On the other hand, use of a continuous wave makes continuous wave
Doppler technology less suited towards measuring a position. As such,
typical continuous wave Doppler devices (i.e., devices in which the FMCW
Doppler is not utilized) were unable to perform position measurement. In
this regard, the present inventors proposed, in JP 2006-14916 A, a
technology which enabled measurement of a position of a tissue within a
living organism, in addition to measurement of the velocity of a tissue
within the living organism, by using FMCW Doppler.

[0007]The FMCW Doppler technology described in the above-noted
publications is a revolutionary technology providing a potential for new
forms of ultrasound diagnosis. The present inventors have continued to
research and improve this landmark technology.

SUMMARY

[0008]The present invention was made in view of the above circumstances,
and advantageously provides a technology for acquiring Doppler
information from a desired position by using a continuous wave.

[0009]In accordance with one aspect of the invention, there is provided an
ultrasound diagnostic apparatus comprising a transmission signal
processing section which outputs a modulated transmission signal that is
a continuous wave having a frequency varied periodically; a
transmission/reception section which transmits ultrasound to a living
organism based on the modulated transmission signal and receives a
reflection wave from the living organism, thereby obtaining a reception
signal; a reception signal processing section which applies a
demodulation processing to the reception signal by using a reference
signal having a waveform that is substantially the same as a waveform of
the modulated transmission signal to obtain a demodulated signal; and a
Doppler information extraction section which extracts Doppler information
from the demodulated signal, wherein a delay processing in accordance
with a depth of a target position within the living organism is performed
to adjust a delay relationship between the reference signal and the
reception signal and the demodulation processing is performed, thereby
selectively extracting the Doppler information from the target position.

[0010]In the above aspect, as the demodulation processing is performed
with respect to a reception signal by using a reference signal, a
demodulated signal containing a signal component having a relatively high
degree of correlation with the reference signal can be obtained. Further,
for performing the demodulation processing, a delay relationship (a
relationship in the time axis direction) between the reference signal and
the reception signal is adjusted in accordance with the depth of a target
position. For example, a phase relationship between the reference signal
and the reception signal is adjusted. Consequently, by aligning the phase
of the reception signal from the target position with the phase of the
reference signal, for example, the reception signal from the target
position can be extracted as a signal component having a relatively high
correlation to the reference signal. In addition, by extracting Doppler
information from the reception signal by using a band-pass filter or a
low pass filter, for example, selective extraction of the Doppler
information from the target position can be achieved. Here, with the
above aspect, it is desirable that the waveform of the reference signal
and the waveform of the modulated transmission signal are completely
identical. However, the reference signal and the modulated transmission
signal may be in a correspondence relationship, in which their waveforms
can be considered to be substantially identical.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]Preferred embodiments of the present invention will be described in
detail based on the following figures, wherein:

[0012]FIG. 1 is a functional block diagram showing the overall structure
of an ultrasound diagnostic apparatus according to the present invention;

[0013]FIG. 2 is a view showing the frequency spectrum of fixed echo and
Doppler echo of a demodulated signal (baseband signal);

[0014]FIG. 3 is a view showing the phase relationship between a reference
signal and a reception signal and the frequency spectrum of a baseband
signal;

[0015]FIG. 4 is a view for explaining dependency of the baseband signal
electric power on depth;

[0016]FIG. 5 is a chart of calculation results indicating a relationship
between the depth at which Doppler echo is generated and a clutter
integral power;

[0017]FIG. 6 is a view showing the frequency spectrum of a reception
signal and noise in invated FMCW Doppler measuring system;

[0018]FIG. 7 is a graph showing calculation results of position
selectivity in accordance with the modulation index;

[0019]FIG. 8 is a chart indicating certain characteristics of one
embodiment of the present invention;

[0020]FIG. 9 is a view for explaining the waveform of a modulation wave;

[0021]FIG. 10 is a diagram showing another preferred embodiment of an
ultrasound diagnostic apparatus according to the present invention;

[0022]FIG. 11 is a view for explaining a relationship among the Doppler
information, depth, and time when the second modulation wave is a
sinusoidal wave;

[0023]FIG. 12 is a view showing a display mode of velocity distribution;

[0024]FIG. 13 is a view for explaining a correspondence relationship
between the Doppler information and the depth when the second modulation
wave is a sawtooth wave; and

[0025]FIG. 14 is a view for explaining a correspondence relationship
between the Doppler information and the depth when the second modulation
wave is a symmetrical triangular wave.

DETAILED DESCRIPTION

[0026]Preferred embodiments of the present invention will be described in
detail with reference to the accompanying drawings.

[0027]FIG. 1 is a functional block diagram showing the overall structure
of an ultrasound diagnostic apparatus according to a preferred embodiment
of the present invention. A transmitting transducer 10 continuously
transmits a transmission wave into a living organism, and a receiving
transducer 12 continuously receives a reflection wave from within the
living organism. Thus, transmission and reception is performed by
different transducers, and transmission/reception by means of a so-called
continuous wave Doppler method is thus executed.

[0028]A power amplifier 14 supplies a power-amplified FM continuous wave
(FMCW) to the transmitting transducer 10. An FM continuous wave (FMCW)
having been subjected to an FM modulation processing using a sinusoidal
wave, for example, is input to the power amplifier 14, and a transmission
wave corresponding to this FM continuous wave is then transmitted from
the transmitting transducer 10. An FM modulator 20 outputs an FM
continuous wave to the power amplifier 14. The FM modulator 20 generates
an FM continuous wave based on an RF wave supplied from an RF wave
oscillator 22 and a modulation wave which is a sinusoidal wave supplied
from an FM modulation wave oscillator 24. The waveform of this FM
continuous wave will be described in detail below with regard to
explanation of the principle.

[0029]A preamplifier 16 applies a reception processing such as low-noise
amplification to a reception wave signal supplied from the receiving
transducer 12 to generate a receiving RF signal, which is output to a
receiving mixer 30. The receiving mixer 30, which is a circuit for
applying orthogonal detection to the receiving RF signal to generate a
complex baseband signal, is composed of two mixers 32 and 34. Each of the
mixers is a circuit which mixes the receiving RF signal with a
predetermined reference signal.

[0030]The reference signal supplied to each mixer of the receiving mixer
30 is generated based on the FM continuous wave output from the FM
modulator 20. Specifically, the FM continuous wave output from the FM
modulator 20 is delayed in a delay circuit 25, and the delayed FM
continuous wave is directly provided to the mixer 32, whereas the delayed
FM continuous wave is supplied to the mixer 34 via a π/2 shift circuit
26 which shifts the phase of the delayed FM continuous wave by π/2.
Consequently, one of the two mixers 32 and 34 outputs an in-phase signal
component (I signal component) and the other outputs a quadrature-phase
signal component (Q signal component). Then, high frequency components of
the in-phase signal component and the quadrature-phase signal component
are removed by LPFs (low pass filters) 36 and 38, respectively, provided
downstream of the receiving mixer 30, so that a demodulated signal having
only a necessary bandwidth after detection can be extracted.

[0031]Here, it is also possible to apply a delay processing to the
sinusoidal modulation wave supplied from the FM modulation wave
oscillator 24 to thereby form a delayed modulation wave, which is then
used for frequency modulation of the RF wave supplied from the RF wave
oscillator 22, thereby generating the reference signal.

[0032]As will be described in detail in the following explanation of the
technological principle of the present invention, a receiving mixer
output signal (i.e. a demodulated signal) which is a result of mixing the
receiving RF signal with the reference signal performed in each mixer
contains a plurality of n-order wave components (n is a natural number
which is 0 or greater) concerning a modulation frequency fm of the
modulation wave supplied from the FM modulation wave oscillator 24.
Specifically, the receiving mixer output signal contains a direct-current
component which is a 0-order wave component, a fundamental component
which is the first-order wave component, and also a plurality of harmonic
components which are n-order wave components where n is 2 or greater. As
such, the demodulated signal containing these plurality of n-order wave
components is output from each of the LPFs 36 and 38.

[0033]FFT circuits (fast Fourier transform circuits) 40 and 42 execute an
FFT operation with respect to each demodulated signal (the in-phase
signal component and the quadrature-phase signal component).
Consequently, the demodulated signal is transformed into a frequency
spectrum in the FFT circuits 40 and 42. Here, the frequency spectrum
output from the FFT circuits 40 and 42 is supplied in the form of
frequency spectrum data with the frequency resolution δf, depending
on the circuit setting condition or the like. The frequency spectrum
output from the FFT circuits 40 and 42 will be described in detail below
with reference to FIG. 2 or the like.

[0034]A Doppler information analyzing section 44 extracts Doppler
information from the demodulated signal which is transformed into the
frequency spectrum. At this time, as the phase relationship between the
reference signal and the reception signal has been already adjusted in
accordance with the depth of a target position within the living organism
by the delay circuit 25, Doppler information from the target position can
be selectively extracted. The relevance between the phase adjustment and
the extraction of Doppler information from the target position will be
described in detail while explaining the principle of the present
invention. The Doppler information analyzing section 44 extracts the
Doppler information for each depth (each position) within the living
organism, to thereby compute the velocity of a tissue within the living
organism for each depth on the ultrasound beam (sound ray), and outputs
the results in real time. Here, the ultrasound beam may be scanned to
thereby compute the velocity of the tissue within the living organism at
each position in two or three dimensional manner.

[0035]A display processing section 46, based on the velocity of the tissue
within the living organism for each depth (position), creates a Doppler
waveform or a graph including information concerning the depth velocity,
for example, and causes a display section 48 to display the Doppler
waveform and the graphs thus created in real time. Here, each of the
sections in the ultrasound diagnostic apparatus as shown in FIG. 1 is
controlled by a system control section 50. Specifically, the system
control section 50 performs transmission control, reception control,
display control, and so on.

[0036]As generally described above, according to the present embodiment,
an ultrasound wave (FMCW) which is obtained by applying FM modulation
using a modulation wave to a continuous wave (CW) is transmitted and
received to obtain a reception signal, and the phase relationship between
the reference signal and the reception signal is adjusted in accordance
with the depth of a target position within the living organism and then a
demodulation processing is performed, so that the Doppler information can
be selectively extracted from the target position. Here, the principle
for selectively extracting the Doppler information from a target position
will be described in detail. First, the fundamental principle of the FMCW
which is used in the present embodiment will be described.

[0037]An FMCW transmission wave obtained by applying FM modulation to a
continuous wave having a frequency f0 by means of a sinusoidal wave
with the modulation frequency fm can be expressed as follows:

[0038]In the above equation 1, Δf is a 0-P value (a zero-peak value:
the maximum frequency deviation) of the frequency variation range, and
β, which is a ratio of the maximum frequency deviation Δf and
the modulation frequency fm, is a modulation index of FM. Further,
the FMCW reception wave with no Doppler shift, when attenuation due to
the living organism is disregarded, can be expressed as follows:

vR(t)=sin {2πf0t+φ0+β
sin(2πfmt+φm)} [Equation 2]

φ 0 = 4 π f 0 d c

:PHASE ANGLE OF RF WAVE f0 CORRESPONDING TO ROUND-TRIP PROPAGATION
TIME 2d/c TO AND FROM TARGET c: PROPAGATION SPEED OF ULTRASOUND

φ m = 4 π f m d c

:PHASE ANGLE OF MODULATION WAVE fm CORRESPONDING TO ROUND-TRIP
PROPAGATION TIME 2d/c TO AND FROM TARGET

[0039]The frequency spectrum of the FMCW transmission wave can be obtained
by expanding Equation 1 by using Bessel series. The FMCW transmission
wave expressed in Equation 1 can be expanded as follows:

[0040]In Equation 3, J0(β), J2n(β), J2n+1(β)
are Bessel function of the first kind. The amplitude of each term is
determined by the modulation index β and the corresponding Bessel
function.

[0041]Further, the frequency spectrum of the reception wave VR(t)
with no Doppler shift can be obtained by expansion of Equation 2. The
FMCW reception wave shown in Equation 2 can be expanded as follows:

[0042]As indicated by Equation 4, the frequency spectrum of a reception
wave and the frequency spectrum of a transmission wave have the same
frequency components. However, the amplitude of each frequency component
of the reception wave varies in accordance with phase differences
φ0 and φm.

[0043]Further, when Doppler shift is included, VR(t) in Equation 2
can be rewritten as follows:

vR(t)=sin {2π(f0+fd)t+φ0+β
sin(2πfmt+φm)} [Equation 5]

[0044]Here, in Equation 5, the Doppler shift with regard to fm, which
is smaller than a shift amount fd of f0, is disregarded.

[0045]The reception waveforms expressed in Equations 2 and 5 described
above are signal waveforms received by the ultrasound transducer (a
receiving RF signal). The ultrasound diagnostic apparatus executes a
demodulation processing with respect to the receiving RF signal. When
demodulating the FMCW receiving RF signal, the demodulation system
multiplies the reference signal by the reception wave, using the FMCW
transmission wave as the reference signal. The receiving mixer output in
the demodulation system is calculated as a result of multiplication of
VT(t) and VR(t) as follows:

[0048]The frequency spectrum of the receiving mixer output which is
expressed by Equation 8, that is the frequency spectrum output from the
FFT circuits 40 and 42 in FIG. 1, are shown in FIG. 2.

[0049]FIG. 2 is a schematic view showing the frequency spectrum of a
demodulated signal. As also indicated in Equation 8, the demodulated
signal includes a plurality of n-order harmonic components (n is a
natural number which is 0 or greater) concerning the modulation
frequency. More specifically, in FIG. 2, the demodulated signal includes
a direct-current component which is the 0-order wave component existing
near the origin O, the fundamental component (fm) which is the first
order wave component, the second harmonic component (2fm) which is
the second order wave component, and the third harmonic component
(3fm) which is the third order wave component. Further, the
demodulated signal also includes the seventh or higher order harmonic
components which are not shown. Each of the n-order wave component
contains a fixed echo 64 and a Doppler echo 62.

[0050]The fixed echo 64 is echo (clutter echo) from a still object which
is a strong reflector within the living organism and is an obstruction
factor when observing the Doppler information. On the other hand, the
Doppler echo 62 is a Doppler signal, which is required. When Doppler
shift is involved, each of the n-order wave components of a Doppler
signal appears on the frequency spectrum in the form of DSB-SC (Double
Sideband-Suppressed Carrier) in which the FM modulation frequency is
suppressed. It should be noted that Equations 1 to 8 described above and
FIG. 2 are described in JP 2000-253949 A.

[0051]The present embodiment is an application of the fundamental
principle of FMCW described above. In the present embodiment, the phase
relationship between the reference signal and the reception signal is
adjusted in accordance with the depth of a target position within a
living organism by means of the delay circuit 25 (a phase shifter or a
delay line) shown in FIG. 1, so that Doppler information from the target
position (i.e. the target depth) can be selectively extracted. More
specifically, by setting the delay time of the delay processing performed
by the delay circuit 25 to a round-trip propagation time of ultrasound
within the living organism, the highest possible degree of correlation
can be obtained between the reception signal from the target depth and
the reference signal, so that only a signal from the target can be
selectively extracted.

[0052]FIG. 3 is a view for explaining the phase relationship (time
relationship) between the reference signal and the reception signal. More
specifically, FIG. 3(a) shows the time waveform of an FMCW transmission
signal (a transmission wave), and FIG. 3(b) shows a reception signal (a
reception wave) corresponding to the transmission signal. The reception
signal is received with a delay time corresponding to τ from the
transmission time. Assuming that the speed of sound is c, the reception
signal obtained from the depth d is received with a delay of τ=2d/c.
Further, in FIG. 3, f0 indicates a carrier frequency (corresponding
to an ultrasound frequency), and fm indicates a frequency for
modulating the carrier (i.e. the modulation frequency). The frequency
spectrum power of the reception signal is shown in FIG. 3(e). Here, while
the frequency spectrum of the transmission signal, when expressed in the
form of power, assumes the same waveform as that shown in FIG. 3(e), the
phase of each power spectrum differs from the transmission spectrum in
accordance with the delay time τ.

[0053]The reference wave (i.e. the reference signal) shown in FIG. 3(c)
which orthogonally-detects the reception signal is a signal obtained by
shifting the transmission wave by a transmission/reception time
difference (i.e. round-trip propagation time τ). The input to the
orthogonal detector, i.e. the reception signal and the reference signal
input to the receiving mixer (indicated by numeral 30 in FIG. 1) will
have a time waveform with the completely identical frequency and phase as
a result of this shift operation. Accordingly, the waveform obtained by
squaring the reception wave (i.e. the reception signal) is output from
the orthogonal detector (i.e. the orthogonal detector output shown in
FIG. 3(d)). Hereinafter, this orthogonal detector output signal will be
referred to as a baseband signal. The baseband signal can be expressed as
a sum of the direct current component with amplitude of 1/2 and an FM
signal with the carrier frequency of 2f0 and the modulation degree
of 2β. As such, with the square of the reception wave, the reception
wave is shifted to the frequency band which is near the direct current
and is double the carrier, as shown by the frequency spectrum of the
baseband signal in FIG. 3(f).

[0054]When the delay time of the reception wave is completely the same as
that of the reference wave, the fundamental component fm and the
harmonic components (see FIG. 2) do not appear in the baseband signal,
and the baseband signal includes only a direct current component as shown
in FIG. 3(f). It should be noted that the components 2f0 and
-2f0 in FIG. 3(f) are to be removed by the LPFs (indicated by
numerals 36 and 38 in FIG. 1) provided downstream of the receiving mixer,
for example. On the other hand, if the delay times do not correspond to
each other between the reception wave and the reference wave, due to this
time difference, harmonic components of the modulation wave, i.e., the
fundamental component fm and the harmonic components (see FIG. 2)
are generated. The harmonic components will be generated even when a time
difference between the reception wave and the reference wave deviates
from "0" by only a slight amount.

[0055]Based on the characteristics described above, according to the FMCW
method of the present embodiment, the phase relationship between the
reception wave (the reception signal) and the reference wave (the
reference signal) is adjusted, whereby the velocity information of a
target tissue can be obtained based on the signal components of the
direct current and near the direct current of the baseband signal. In
this sense, the ultrasound diagnostic apparatus according to the present
embodiment can be referred to as a phase shift FMCW ultrasound Doppler
system.

[0056]FIG. 4 is a view for explaining the dependency of the direct current
and harmonic components of a baseband signal on the depth within a living
organism (i.e. the distance from the body surface). FIG. 4 shows the
frequency spectra of a baseband signal (a demodulated signal) for each of
a plurality of modulation indexes β. The frequency spectrum for each
modulation index β shown in FIG. 4 corresponds to a frequency
spectrum obtained by adding a dimension in the distance direction to the
representation of the frequency spectrum shown in FIG. 2. Here, the each
frequency spectrum concerning from β=0 to β=:30 represents a
reflection power from a fixed tissue and moving target. In FIG. 4, the
attenuation effect in the tissue is not considered. Here, the magnitude
of the FM modulation degree is generally quantified by the modulation
index β. Further, β is defined as a ratio between the maximum
frequency deviation Δf of the carrier wave due to FM modulation and
the modulation frequency fm, and is defined by
β=Δf/fm.

[0057]When β=0, meaning that no modulation is performed, the system
of the present embodiment is equivalent to a general CW Doppler velocity
measurement system. In such a case, the dependency on positions is not
found in the reflection powers from any depths. Further, as no modulation
is performed for transmission and reception, there is no possibility that
a modulation wave component appears in the baseband signal. When FM
modulation is applied to a continuous ultrasound (CW) and the frequency
deviation Δf is gradually increased, the FMCW
transmission/reception wave will be an FM modulated signal, with its
power being shifted from the carrier wave to the sideband wave. If there
is no difference in the delay time between transmission and reception, a
harmonic component of the modulation wave is not generated in the
baseband signal (see FIG. 3). In order to prevent a time difference
between two inputs to the orthogonal detector (i.e. the reception signal
and the reference signal), a delay time corresponding to the delay time
of the reflection wave from the target depth may simply be applied to the
reference signal.

[0058]The example illustrated in FIG. 4 shows a case in which the delay
time to be applied to the reference wave is set to correspond to the
delay time of the reception wave when the distance is d0=7.5 cm.
Consequently, the reflection power from the distance d0 includes
only a direct current component, and no harmonic components of the
modulation wave are generated. At positions other than d=d0, the
degree of correlation between the two signals is less because a time
difference is caused between the transmission wave and the reception
wave, which in turn results in a decrease in the direct current component
and simultaneously resulting in appearance of harmonic components. This
phenomenon can be understood by the concept that, when a time difference
is caused between (b) and (c) of FIG. 3, a phenomenon in which a
difference in the instantaneous frequencies is repeated in a cycle of the
modulation frequency occurs.

[0059]As the modulation degree (i.e. the modulation index β)
increases, the reflection power at the distance d0 varies more
sensitively with respect to a position deviation. In other words,
selectivity of the reflection power at the distance d0 increases.
This tendency becomes more and more noticeable as β increases. FIG.
4 shows this tendency when β is 0 to approximately 30. When β
is 30 or greater, this selectivity is close to a function similar to the
role of the range gate in the PW (pulse wave) Doppler velocity measuring
system.

[0060]It is now assumed that the reflection power from the distance
d0 involves Doppler shift fd. In this case, while the distance
dependency similar to that of the fixed tissue appears, the distance
dependency appears with a shift from the direct current component by the
Doppler frequency fd. While the Doppler spectrum also appears in
both sideband waves of the modulation wave and the harmonic wave, the
Doppler echo from the distance d0 includes only a frequency
component which is shifted from the direct current component by fd,
as shown in FIG. 4(DP).

[0061]As shown in FIG. 4(DP), the Doppler echo from the distance d0
includes only the frequency component which is shifted from the direct
current component by fd, because the correlation between the
reference signal and a Doppler signal is maximized at the frequency which
is shifted from the carrier frequency f0 by fd. This Doppler
signal appears only near the direct current components and does not
appear near the harmonic components (fm, 2fm, 3fm, . . . )
of the modulation wave. Accordingly, by extracting this component near
the direct current component by means of a low pass filter, the Doppler
information in which position information is specified can be obtained
with SNR (signal-to-noise ratio) similar to that of general CW Doppler
system being maintained.

[0062]More specifically, by applying a delay processing to the reference
signal with a delay time τ (τ=2d/c, c: velocity of sound, d:
depth) corresponding to the target depth (position) d by means of the
delay circuit 25 shown in FIG. 1, the FFT circuits 40 and 42 of FIG. 1
output the Doppler frequency spectrum information corresponding to FIG.
4(DP), and the Doppler information analyzing section 44 of FIG. 1
extracts the Doppler signal near the direct current component of FIG.
4(DP).

[0063]Next, several characteristics concerning the ultrasound diagnostic
apparatus (the phase shift FMCW ultrasound Doppler system) according to
the present embodiment will be described.

[0064]Clutter Power

[0065]As the degree of specification of depth (position) information
depends on the modulation degree (i.e. the modulation index) β, it
is necessary to set β to a great value in order to ensure the
necessary position resolution. With β being set to a large value,
the position selectivity becomes sharp. Specifically, as shown in FIG. 4,
as β increases, a sharp waveform can be obtained at the target
distance d0. Consequently, only the reflection power near the
distance d0 is significantly reflected in the reception signal. This
nature is common for a fixed target and a moving target.

[0066]In contrast to the conventional CW method in which the clutter power
is a sum of the reflection powers from all the fixed targets on the sound
ray, according to the method (the phase shift FMCW) of the present
embodiment, in which only the clutter from a selected position is
generated, the clutter power can be reduced to an extreme degree.

[0067]FIG. 5 is a chart of calculation results showing a relationship
between the depth (i.e. the depth from the body surface) at which the
Doppler echo is generated and the integrated clutter power, with the
parameter being the FM modulation index (β). FIG. 5 shows the
calculation values of the integrated clutter power 84 and the Doppler
signal power 82 for each FM modulation index (β). Further, the
clutter improvement degree 90 indicates a reduction in the clutter
integral power in comparison with the conventional CW. FIG. 5 shows a
tendency in which as the modulation-degree (i.e. the modulation index)
β increases, the integrated clutter power is generally decreases.
For example, with β=100, the clutter power can be reduced by about
25 dB less than that in the conventional CW.

[0068]Signal-to-Noise Ratio (SNR)

[0069]FIG. 6 shows the spectrum of a reception signal in the RF frequency
band and the baseband frequency band concerning the Doppler velocity
measurement system of each of the PW (pulse wave), conventional CW, and
FMCW. The noise dominating the SNR of PW Doppler is determined by a
signal bandwidth at the time of sampling after the orthogonal detection.
As in communication systems, this noise can be treated as a white Gauss
noise which is distributed in the bandwidth of RF signals. The signal
bandwidth at the time of sampling is given by a reciprocal of the pulse
width in the baseband. If the pulse width is assumed to be 2 μsec.,
for example, the bandwidth will be 500 kHz.

[0070]On the other hand, in the conventional CW Doppler velocity
measurement system, PRF does not exist, and the signal bandwidth is
approximately the maximum frequency of the Doppler signal to be measured.
If this value is 5 KHz, for example, as the difference in SNR between PW
and CW can be expressed by the difference between the bandwidths, the
signal bandwidth of CW will be 500 kHz/5 kHz=100. As such, the SNR of CW
is improved by 20 dB in comparison with that of PW.

[0071]The signal bandwidth of FMCW is determined by the maximum frequency
of the Doppler signal in the baseband signal. In FMCW, however, the
frequency band which depends on the modulation degree is included in the
RF (ultrasound) frequency band, and a signal diffuses in a wide band.
However, as this wide band signal is compression-transformed into a
narrow band baseband signal near the direct current component due to
orthogonal detection, the noise is also in the narrow band, leading to a
significant improvement of SNR. The degree of improvement is similar to
the case of CW, and in the case of 500 kHz/5 kHz=100 as described above,
the improvement of 20 dB compared to the PW can be expected.

[0072]Position Selectivity (Position Resolution)

[0073]With the PW Doppler, the Doppler information from a specific
position can be obtained, however, with the normal CW Doppler method, the
position information cannot be obtained.

[0074]The phase shift FMCW system of the present embodiment is considered
to complement these characteristics. In the demodulation process of the
present embodiment, the delay time of the reference signal for use in
orthogonal detection is matched with the distance to the target, so that
the reference signal assumes the position selectivity. This selectivity
has a nature that as the modulation index β increases, the
selectivity is improved. With the increase in the modulation index
β, the bandwidth of the carrier band is expanded. The necessary
bandwidth is approximately given by the following equation:

BW≈2(fm+Δf)=2(fm+βfm)=2fm(1+β)
[Equation 9]

[0075]FIG. 7 is a chart showing a calculation result of the position
selectivity in accordance with the degree of modulation index. The half
width shown in FIG. 7 means a half width of the frequency spectrum
waveform at a target position (i.e. a half width of the frequency
spectrum waveform at the distance d0 of each frequency spectrum
shown in FIG. 4).

[0076]As shown in FIG. 7, by increasing β, the position selectivity
can be set to several mm or less. This result indicates that the wider
the frequency band of the carrier wave, the more improved the position
resolution of the target. This feature is not inconsistent with the
feature concerning PW, that the smaller the pulse width, the greater the
improvement in the position resolution in RADAR systems.

[0077]As such, according to the phase shift FMCW method of the present
embodiment, by setting β to a sufficiently large value which
satisfies the desired position resolution, i.e. by setting the bandwidth
of FMCW given by Equation 9 to approximately the same bandwidth of PW,
Doppler information with position information and a desired SNR can be
obtained.

[0078]Maximum Blood Flow Velocity

[0079]In the FMCW method, as in the PW method, the maximum velocity is
limited. In the FMCW method, the carrier wave is modulated with a
modulation wave having a frequency of fm, and the maximum measurable
frequency is limited to fm/2 due to aliasing. This limitation can be
reduced because aliasing doppler signals are smaller than required
doppler signals in the FMCW method.

[0080]Structure of Transmission/Reception Section

[0081]When the average power in the probe input is equal among the PW, CW,
and FMCW Doppler velocity measurement systems, the peak power for
achieving the improvement effects with regard to the clutter and noise
described above can be reduced in the FMCW method of the present
embodiment to approximately 1/100 compared with PW, and the peak voltage
can be reduced to 1/10. On the other hand, when the transmission power
which can be input to the probe is limited by the peak value, in contrast
to PW, with the FMCW of the present embodiment, suppression of the
clutter and improvement in SNR can be further expected at a ratio which
can increase the power, in addition to the improvement effects with
regard to the clutter and noise.

[0082]A preferred embodiment of the present invention has been described
above; some of the characteristics (advantages) of this embodiment are
summarized as shown in FIG. 8.

[0083]Further, in the embodiment described above, the sinusoidal wave as
shown in FIG. 9 is utilized as the modulation wave. In place of this
sinusoidal wave, a symmetrical triangular wave as shown in FIG. 9 may be
used as the modulation wave. The use of the symmetrical triangular wave
is advantageous in that the position information and the Doppler
information can be measured separately (see JP 2006-14916 A). Here, as
the symmetrical triangular wave has a periodicity which is similar to
that of the sinusoidal wave, even if the symmetrical triangular wave is
used in place of the sinusoidal wave, the phase shift FMCW ultrasound
Doppler system can be configured.

[0084]In addition, as shown in FIG. 9, a symmetrical triangular wave in
which the frequency change at the conversion point of the modulation
frequency change is smoothed in terms of time (i.e. a corner-rounded
symmetrical triangular wave) may also be used as the modulation wave.
This waveform provides an advantage that high frequency components
generated by the symmetrical triangular wave at the conversion point of
the modulation frequency change can be removed. In other words, by
smoothing the frequency change at this conversion point in terms of time,
generation of the high frequency components can be reduced, such that
excessive expansion of the RF bandwidth can be advantageously prevented.

[0085]Also, as an example variation for adjusting the phases between a
reception signal and a reference signal, the delay circuit 25 may be
displaced to a position immediately before the power amplifier 14 (i.e.
the position indicated by P in FIG. 1) in FIG. 1. In other words, it is
possible to apply a delay processing to the transmitting FMCW to be
supplied to the power amplifier 14 while applying no delay processing to
the reference signal to be supplied to the receiving mixer 30, and to
then adjust the phases between the reference signal and the reception
signal. Here, a further delay circuit 25 may be provided immediately
before the power amplifier 14 while the delay circuit 25 shown in FIG. 1
remains provided. Specifically, it is also possible to apply a delay
processing to the transmitting FMCW to be supplied to the power amplifier
14 and further apply a delay processing to the reference signal to be
supplied to the receiving mixer 30, and then adjust the phases between
the reference signal and the reception signal.

[0086]In addition, it is also possible to delay the modulation signal
(i.e. the modulation wave) for use in the frequency modulation processing
by a delay amount in accordance with the depth of a target position to
thereby generate a delayed modulation signal, and then perform a
frequency modulation processing with respect to the carrier signal (an RF
wave) by using the delayed modulation signal to form a reference signal.
Then, the phases of the reception signal and the reference signal are
adjusted. In this case, a modulated transmission signal (an FMCW
transmission signal) is generated by applying the frequency modulation
processing to the carrier signal by using a modulation signal which is
not delayed, for example.

[0087]With the ultrasound diagnostic apparatus shown in FIG. 1, in which
the phase relationship between the reference signal and the reception
signal is adjusted in accordance with the depth of a target position
within the living organism by the delay circuit 25, the Doppler
information from the target position can be selectively extracted.
Accordingly, by extracting the Doppler information for each depth (each
position) within the living organism, the velocity of a tissue within the
living organism can be calculated for each depth on the ultrasound beam
(ray of sound), for example. In addition, with the periodic variation in
the delay amount in the delay circuit 25, the target position is
periodically shifted within a subject section along the ultrasound beam,
so that the Doppler information can be extracted from a plurality of
positions through the subject section, as will be described below.

[0088]FIG. 10 is a functional block diagram showing the overall structure
of an ultrasound diagnostic apparatus according to another preferred
embodiment of the present invention. The ultrasound diagnostic apparatus
shown in FIG. 10 is an improved version of the ultrasound diagnostic
apparatus shown in FIG. 1, and differs from the apparatus shown in FIG. 1
in that a second modulation wave oscillator 52 is provided. Hereinafter,
the ultrasound diagnostic apparatus shown in FIG. 10 will be described
mainly with regard to the additional advantages achieved by the provision
of the second modulation wave oscillator 52, and description of elements
corresponding to those in the apparatus of FIG. 1 will not be repeated.

[0089]In the ultrasound diagnostic apparatus shown in FIG. 10, as in the
ultrasound diagnostic apparatus shown in FIG. 1, the phase relationship
between the reference signal and the reception signal is adjusted in
accordance with the depth of a target position within the living organism
by means of the delay circuit 25, and the Doppler information from the
target position can be selectively extracted. More specifically, by
setting the delay time (the delay amount) of the delay processing
performed by the delay circuit 25 to the propagation time required for
the ultrasound to travel a round trip through the living organism, the
correlation between the reference signal and the reception signal from
the target depth is maximized, so that a signal from just the target can
be selectively extracted.

[0090]In FIG. 10, the delay time in the delay circuit 25 is periodically
varied to periodically shift the target position within a subject section
along the depth direction, whereby the Doppler information is extracted
from a plurality of positions throughout the subject section. Here, the
delay circuit 25 periodically varies the delay time based on a signal
output from the second modulation wave oscillator 52.

[0091]The second modulation wave oscillator 52 outputs a second modulation
wave having a frequency which is lower than that of the modulation wave
(i.e. the first modulation wave) output from the FM modulation wave
oscillator 24. If the frequency of the first modulation wave is set to
approximately 5 kHz, for example, the frequency of the second modulation
wave may be set to approximately 50 Hz, for example. The delay circuit 25
uses this second modulation wave with a relatively low frequency to
periodically vary the delay time.

[0092]With the periodic variation of the delay time, the target position
at which the correlation between the reference signal and the reception
signal is the maximum also varies periodically along the ultrasound beam
direction (i.e. the depth direction in the living organism).
Specifically, the target position is periodically shifted within a
certain range (i.e. within the subject section) along the depth
direction, so that the Doppler information can be extracted from a
plurality of positions through the subject section.

[0093]Here, in FIG. 10, as in the case with FIG. 1, the shift circuit 25
may be displaced to a position immediately before the power amplifier 10
(i.e. the position indicated by P in FIG. 10). In other words, it is
possible to apply a delay processing with a periodic variation to the
transmitting FMCW to be supplied to the power amplifier 14 while applying
no delay processing to the reference signal to be supplied to the
receiving mixer 30, and then adjust the phases of the reference signal
and the reception signal.

[0094]The target position which is periodically shifted is determined in
accordance with the delay time (delay amount) in the delay circuit 25.
The system control section 50 confirms the target position which
periodically shifts based on the second modulation wave output from the
second modulation wave oscillator 52 or based on the delay time in the
delay circuit 52. The system control section 50 then associates the
Doppler information (e.g. the Doppler shift amount, the electric power of
Doppler components, and so on) obtained in the Doppler information
analyzing section 44 with the position (depth) from which the Doppler
information is obtained. This correspondence relationship is then
provided to the display processing section 46, and so on.

[0095]FIG. 11 is a view for explaining the correspondence relationship
between the Doppler information and the depth when the second modulation
wave is a sinusoidal wave. FIG. 11 shows a graph in which the time axis
and the depth axis are provided on the bottom surface and the Doppler
information (the Doppler shift amount or the electric power of Doppler
components) is indicated in the Z axis in the height direction.

[0096]When the second modulation wave is a sinusoidal wave, the delay time
in the delay circuit 25 (FIG. 10) varies in a sinusoidal wave form with
elapse of time, and the target position (depth) from which the Doppler
information can be obtained also varies in a sinusoidal wave form with
elapse of time.

[0097]FIG. 11 shows a state in which, with elapse of time, the target
position varies along the sinusoidal wave between the depth -d/2 and the
depth +d/2 (i.e. the subject section). The graph in FIG. 11 shows an
example case in which the frequency of the second modulation wave is
fm2, in which case the target position varies along the sinusoidal
wave having a period of 1/fm2. Further, at each of the positions
which vary along the sinusoidal wave, the Doppler information obtained
from the corresponding position is indicated in the Z-axis direction.

[0098]As described above, as the Doppler information can be obtained for
each position within the subject section from the depth -d/2 to the depth
+d/2, the velocity distribution 70 within the subject section from the
depth -d/2 to the depth +d/2 can be obtained as shown in FIG. 11, by
calculating the velocity for each position (depth) from the Doppler
information.

[0099]It should be noted that, in FIG. 11, a relatively great amount of
Doppler information is obtained at an intermediate position between the
depth -d/2 and the depth +d/2. In the blood vessel, a larger amount of
blood flow can be obtained at the center of the blood vessel than in the
vicinity of the blood vessel wall. Accordingly, by setting the ultrasound
beam so as to be orthogonal to the blood vessel and setting the subject
section (from the depth -d/2 to the depth +d/2) in a portion on the
ultrasound beam corresponding to the blood vessel, the measurement result
as shown in FIG. 11, for example, can be obtained, and the velocity
distribution 70 of the blood flow within the blood vessel can be
obtained.

[0100]FIG. 12 is a view showing a display mode of the velocity
distribution, and shows a graph in which the time axis and the depth axis
are provided on the bottom surface and the velocity obtained from the
Doppler information is indicated in the height direction. The graph shown
in FIG. 12 is an example of an image formed by the display processing
section 46 and displayed on the display section 48 (see FIG. 10). The
display processing section 46 forms the image shown in FIG. 12 based on
the correspondence relationship between the Doppler information and the
position (depth) obtained from the system control section 50, for
example.

[0101]Specifically, the target position (depth) which is shifted with
elapse of time and the Doppler information obtained from the Doppler
information analyzing section 44 of FIG. 10 are associated with each
other, whereby the velocity (i.e. the flow velocity) is calculated from
the Doppler information. Then, with the repetitious shifting of the
target position within the subject section from the depth -d/2 to the
depth +d/2 with elapse of time, the graph shown in FIG. 12 showing the
variation in the flow velocity with time within the subject section is
formed.

[0102]The graph of FIG. 12 shows a state in which the flow velocity
distribution within the subject section from the depth -d/2 to the depth
+d/2 varies with elapse of time. For example, by setting the ultrasound
beam so as to be orthogonal to the blood vessel and setting the subject
section (from the depth -d/2 to the depth +d/2) in a portion on the
ultrasound beam corresponding to the blood vessel, the measurement result
as shown in FIG. 12 can be obtained as a variation of the flow velocity
distribution of the blood flow in that blood vessel portion.

[0103]Here, while a case in which the second modulation wave is a
sinusoidal wave has been described with references to FIGS. 11 and 12,
the second modulation wave may be a sawtooth wave or a triangular wave.

[0104]FIG. 13 is a view for explaining the correspondence relationship
between the Doppler information and the depth when the second modulation
wave is a sawtooth wave. Specifically, FIG. 13, similar to FIG. 11, shows
a graph in which the time axis and the depth axis are provided on the
bottom surface and the Doppler information is indicated in the height
direction. When the second modulation wave is a sawtooth wave, the delay
time in the delay circuit 25 (FIG. 10) varies in a sawtooth wave shape
with elapse of time, and the target position (depth) from which the
Doppler information is obtained also varies in a sawtooth wave shape with
elapse of time.

[0105]FIG. 13 shows a state in which the target position varies along the
sawtooth wave between the depth -d/2 and the depth +d/2 (i.e. the subject
section) with elapse of time. Here, in the graph shown in FIG. 13,
similar to the graph shown in FIG. 11, the frequency of the second
modulation wave is fm2, and the target position varies along the
sawtooth wave having a frequency of 1/fm2. Then, at each of the
positions which vary along the sawtooth wave, the Doppler information
obtained from the corresponding position is indicated in the height
direction. Additionally, as in the case of FIG. 11, the velocity
distribution 70 within the subject section from the depth -d/2 to the
depth +d/2 may be obtained.

[0106]FIG. 14 is a view for explaining the correspondence between the
Doppler information and the depth when the second modulation wave is a
symmetrical triangular wave. Specifically, FIG. 14, similar to FIGS. 11
and 13, shows a graph in which the time axis and the depth axis are
provided on the bottom surface and the Doppler information is indicated
in the height direction. When the second modulation wave is a symmetrical
triangular wave, the target position (depth) from which the Doppler
information is obtained varies along a symmetrical triangular wave shape
with elapse of time. In FIG. 14, at each of the positions which vary
along the symmetrical triangular wave, the Doppler information obtained
from the corresponding position is indicated in the height direction.
Further, as in the cases of FIGS. 11 and 13, the velocity distribution 70
within the subject section from the depth -d/2 to the depth +d/2 may be
obtained.

[0107]While example preferred embodiments of the present invention and
some modification examples have been described, the preferred embodiments
or the like described above are provided only for illustrative purposes,
and therefore do not limit the scope of the present invention. It should
therefore be understood that the present invention includes various
modifications within the range of the nature of the present invention.

[0108]For example, in the embodiments described above, when forming a
modulated transmission signal that is a continuous wave having a
frequency periodically varied, a frequency modulation processing is
applied to a carrier wave signal (i.e. an RF wave supplied from the RF
wave oscillator 22). In place of this frequency modulation processing, a
phase modulation processing (PM processing), which is obvious to a person
with ordinary skill in the art as an angle modulation method similar to
the frequency modulation processing, may alternatively be used. More
specifically, a waveform which is the same as or equivalent to the FM
continuous wave output from the FM modulator 20 may be formed by applying
the phase modulation processing to the carrier signal (i.e. an RF wave
supplied form the RF wave oscillator 22). Here, it is possible to store
the data of a continuous wave with periodically varying frequencies in a
memory or the like and generate the continuous wave based on the data
which is read from this memory.

[0109]That is, while the preferred embodiments of the present invention
have been described using specific terms, such description is for
illustrative purposes only, and it is to be understood that changes and
variations may be made without departing from the spirit or scope of the
appended claims.